1. Field of the Invention
The subject invention relates to the use of polyisocyanate-polyol mixtures to stabilize rock and coal formations.
2. Description of the Prior Art
Processes for stabilizing geologic formations or sealing formations against water or gas with the aid of polyurethanes are known in the art.
For instance, German Pat. No. 1,129,894 (British Pat. No. 885,762) describes how mixtures of polyisocyanates and a polyol containing at least three reactive hydroxy groups in a liquid carrier are pressed into the formation which is to be sealed or stabilized. Suitable polyols include polyalcohols, such as hexane triol, polyester polyols of dicarboxylic acids and bi- and trifunctional alcohols, and preferably, castor oil.
The process has drawbacks because of the low elasticity or pressure and bending strength of the polyurethane used for the stabilization of the geologic formations. Consequently, shifts in the stabilized formations can occur as a result of the pressure of the formations and the effects of external forces such as are incurred during the mechanical mining of coal. Another drawback is the nonmiscibility of castor oil with water. Consequently, the castor oil must be separated from the system by absorbing water. Moreover, the relatively high viscosity of the castor oil (approximately 1000 mPas at 25.degree. C.) causes an increase of the viscosity of the system so that a flawless penetration into the smallest cracks of the geological formation can no longer be guaranteed. Another drawback of the process is the low bonding between the polyurethane and coal or stone when the system contains significant quantities of castor oil.
In order to avoid these drawbacks, German Published Application No. 1,745,366 (British Pat. No. 1,104,839describes polyol mixtures of a tree resin with a softening point of at least 70.degree. C. or one of it derivatives and a low molecular polyol with a hydroxyl number of at least 1000 such as trimethylolpropane, glycerine, hexanetriol, sorbitol, cane sugar and others. The polyurethane foams produced in this manner have excellent tenacity. By using the above-mentioned tree resins, however, systems are obtained which have a relatively high viscosity.
German Pat. No. 2,436,029 (U.S. Pat. No. 4,114,382) describes the use of a polyisocyanate-polyol mixture for stabilizing geological formations wherein the polyol component contains 5 to 50 percent by weight of a polyether polyol of alkylene oxide with a hydroxyl number below 100. As illustrated in the application, this results in highly flexible polyurethanes with a high elasticity modulus and high bending strength as well as high bonding to coal and surrounding rock formations. However, according to German Published Application No. 1,745,366, very brittle polyurethane foams are obtained from the same polyol mixtures.
As can be seen, there is a need for polyurethane systems for stabilizing rock and coal formations by bonding these formations to themselves or other geological formations. These systems must have a relatively low viscosity so they can penetrate into gaps and smallest cracks, and must have high adhesion. In this manner, they guarantee sufficient coating of the surface.